A machine may be used to manufacture packages, in particular packages for fresh meat products, which on the one hand should have a long shelf life and on the other hand should still look attractive to customers in the sales display. To make a fresh meat product look attractive, a corresponding oxygen content at the product surface is required to achieve the attractive reddening.
To achieve this, there are two well-known methods: On the one hand, one lid foil can consist of a material that is impermeable to oxygen, and the interior of the package is (re)gassed with oxygen before it is sealed, thus taking care that there is an oxygen content of more than 60% after the package has been closed. This high oxygen concentration provides a long shelf life as well as a permanent reddening of the product surface.
An alternative method consists in the use of a tray filled with the product which is sealed with two lid foils, one outer lid foil impermeable to oxygen, and one inner lid foil permeable to oxygen, wherein the interior of the package is evacuated and the product achieves a long shelf life thanks to the low residual oxygen. The discoloration occurring in this case is optically imperfect, but it is corrected before the package is placed in the sales display by removing the outer lid foil impermeable to oxygen as now oxygen can diffuse into the package through the inner lid foil permeable to oxygen, resulting in a renewable reddening of the product mainly in the visible region.
Both methods are preferably suited for a combination of package and product, in which the product as a whole is located below the upper edge (equivalent to the sealing level) of the package tray into which it was filled. If a portion of the product surface would be located above the sealing level, the lid foil would contact the product there and the oxygen from inside the package, or also the oxygen diffusing from outside, would not be able to reach these surfaces. Thus, undesired discolorations are formed there at the product which the buyer of this product judges as being imperfect.
The document EP 0 690 012 B1 describes a two-layered foil with an outer layer impermeable to oxygen and an inner layer permeable to oxygen which is suited to diffuse oxygen to the product in spite of a product contact. This effect is generated by particles, preferably food starch particles, being located between the two layers as uniformly distributed as possible, the particles taking care that oxygen can move in this intermediate area and thus continuously continue flowing to the points where oxygen is consumed due to product contact. The oxygen altogether consumed in this intermediate area is enriched again by the quantity of oxygen inside the package and by the lid foil permeable to oxygen.
Such multilayer foils often also have shrink properties that require stability in trays which is provided by the common rolled edge in trays. Therefore, today tray sealers are known which seal trays filled with projecting products with said two-layered lid foils.
A disadvantage of tray sealers compared to deep-drawing packaging machines is the cost for stock-keeping of the trays to be processed, the missing value added in own production due to the purchase of these trays as well as the possible higher power density of the number of packages per time unit.
To let the shrinking capacity of the lid foils become effective, it must be heated to a minimum temperature before it is sealed with the tray and before the chamber is subsequently aerated, so that in this state the foil lies against the product in partial areas as “a second skin” and thus represents a very attractive package. It is well-known to the person skilled in the art to heat for this purpose lid foils in a sealing station by placing them against a plate to achieve the required temperature which makes the shrinking properties become effective. However, this temperature is below the sealing temperature of the sealing plate or the foil, respectively.
A complex and expensive solution is a separate heating means disposed in the plate, which on the one hand requires space and on the other hand requires efforts for leading the energy line for the heating means outside in such a way that the tightness of the chamber in the closed state, e.g. for the evacuation process or sealing, is not annulled.
Simpler constructions provide a plate which is to be brought to the desired temperature only by the radiant heat of the sealing plate. This is mainly disadvantageous when the packaging machine is being started, for example after it has been reset for a new package or a new product, as a very long waiting time results. Moreover, this process is not controlled but requires that then the radiant heat corresponds to the heating of the plate in the subsequent running operation.